1. Field of the Invention
The present invention relates to a driving method of a semiconductor device. The present invention relates to a driving method of a semiconductor device including a non-volatile memory element in which data can be written and erased.
Note that in this specification, a semiconductor device refers to a device which can function by utilizing semiconductor properties.
2. Description of the Related Art
Non-volatile memory elements are semiconductor elements which can hold data semi-permanently by electrical or physical operation. Non-volatile memory elements can hold a plurality of storage states utilizing the fact that the threshold voltage changes depending on the amount of charge accumulated in charge accumulation layers.
The threshold voltage of a non-volatile memory element is shifted in a positive direction when electrons are accumulated a lot in a charge accumulation layer, and shifted in a negative direction when electrons are discharged from the charge accumulation layer. The data of a non-volatile memory element is written or read by referring the threshold voltages thereof while applying voltage to wirings connected to the non-volatile memory element.
A semiconductor device including a non-volatile memory element read out data by referring the shift of a threshold voltage; however, it is possible that electrons are not sufficiently accumulated or discharged in/from a charge accumulation layer. When electrons are not sufficiently accumulated or discharged in/from a charge accumulation layer, a semiconductor device cannot correctly read out a storage state held in non-volatile memory elements (e.g., see Patent Document 1).